A fluorinated polymer having carboxylic acid type functional groups or sulfonic acid type functional groups is known as a material for ion-exchange membranes (ion exchange membranes for electrolysis of an alkali chloride, electrolyte membranes for fuel cells, etc.).
Such a fluorinated polymer is obtainable, for example, by polymerizing a fluorinated monomer having a carboxylic acid type functional group or a sulfonic acid type functional group, and tetrafluoroethylene (hereinafter referred to TFE), in a polymerization medium in the presence of a polymerization initiator, in a container capable of polymerizing a monomer in a pressurized state.
As the polymerization initiator, usually an azo compound or an organic peroxide is used (for example, Patent Document 1).
However, with an azo compound, the polymerization rate is slow, and therefore, when an azo compound is used as a polymerization initiator, the productivity of a fluorinated polymer tends to be low. Further, in the case of using an azo compound as a polymerization initiator, the molecular weight of an obtainable fluorinated polymer tends to be low.
On the other hand, with an organic peroxide, the polymerization rate is fast as compared with the azo compound, and therefore, when an organic peroxide is used as a polymerization initiator, the productivity of a fluorinated polymer will be better as compared with the case of using the azo compound. Further, in the case of using the organic peroxide as a polymerization initiator, the molecular weight of an obtainable fluorinated polymer will be relatively high.
However, a usually available organic peroxide has a low molecular weight and is likely to be easily volatilized. Therefore, when a fluorinated monomer having a relatively high boiling point and TFE having a relatively low boiling point are polymerized, the concentration of the organic peroxide and TFE in the gas phase in the container tends to be high. As a result, in the gas phase in the container, TFE tends to be mainly polymerized to form a fluorinated polymer having an extremely high proportion of structural units based on TFE, as an impurity.
As a method for producing a fluorinated polymer, whereby the molecular weight of the obtainable fluorinated polymer is relatively high, and a fluorinated polymer having an extremely high proportion of structural units based on TFE is less likely to be formed as an impurity, the following methods have been proposed.
(1) A method of polymerizing a fluorinated monomer having a sulfonic acid type functional group, and TFE, at from 0 to 35° C. in a polymerization medium in the presence of [F(CF2)4-10COO]2 or [CF3CF2CF2O(CF(CF3)CF2O)0-8CF(CF3)COO]2 as a polymerization initiator in a container capable of polymerizing a monomer in a pressurized state (Patent Document 2).
(2) A method of polymerizing a fluorinated monomer having a sulfonic acid type functional group, and TFE, at from 25 to 45° C. in a polymerization medium in the presence of [CF3CF2CF2O(CF(CF3)CF2O)0-8CF(CF3)COO]2 as a polymerization initiator in a container capable of polymerizing a monomer in a pressurized state (Patent Document 3).
In the methods (1) and (2), an organic peroxide is used as a polymerization initiator, and the polymerization temperature is relatively low, whereby the molecular weight of an obtainable fluorinated polymer is relatively high. Further, the organic peroxide used in the methods (1) and (2) has a high molecular weight, whereby the concentration in the gas phase in the container is less likely to be high, and a fluorinated polymer having an extremely high proportion of structural units based on TFE is less likely to be formed as an impurity.
However, the organic peroxide used in the methods (1) and (2) is hardly available and is not suitable for industrial production.